This invention relates to coated granules. In particular, this invention relates to porous granules containing liquid material held therein for slow release by a porous coating covering the opening of each pore. The invention further relates to a process for preparing such coated granules. The effect of the coating is to delay the rate of release of the liquid material to the surrounding medium. The coating thus serves to increase the useful life of the granules as a means for dispersing the material into the surrounding medium. By limiting the maximum rate of diffusion from the pores, the coating also helps to prevent the usual occurrence of a rapid initial increase and subsequent rapid decrease in the concentration of the material in the surrounding medium. A flatter curve of concentration vs. time outside the granule is thus achieved.
The use of membranes, coatings, and capsules for the controlled release of liquid materials is well known in the art of both agricultural and non-agricultural chemicals. In the agricultural area, such slow release techniques have helped improve the efficiency of herbicides, insecticides, fungicides, bactericides, and fertilizers. Coating technology in the agricultural area is manifest in the form of coated droplets, such as microcapsules, coated solids such as porous or non-porous particles, and coated aggregates of solid particles. Non-agricultural uses include encapsulated dyes, inks, pharmaceuticals, flavoring agents, and fragrances. In some instances, a water soluble encapsulating material is desired, whereby the encapsulated material is immediately released upon contact of the capsule with water. Some coatings are designed to release the entrapped liquid upon the application of external pressure to break or crush the coating.
The above coatings completely enclose the material held inside, and prevent any release of the material until the coating is broken, dissolved, or otherwise removed. Other coatings are porous in structure, permitting a slow rate of diffusion of the entrapped material to the surrounding medium. This type is particularly effective in the agricultural area. Water insoluble coatings are particularly useful in agriculture, especially when the surrounding medium is water itself, a water-containing material such as soil, or air in areas of frequent rainfall.
Porous granules offer distinct advantages for chemicals in a wide variety of commercial applications by improving the ease of handling as well as the ease of distributing or dispersing the chemicals over a wide area, and by offering slow release characteristics inherent in the porous structure of the granule itself. In addition, granules of high pore volume are capable of retaining a considerable volume of liquid inside their pores, with only a small fraction initially exposed to the outer medium.
The use of coatings of the external granule surface to further enhance the release delaying characteristics of the granules is known. Certain problems are encountered when conventional techniques are used, however. Adhesion of the coating to the surface of the granule is often a problem. Adhesion can be improved by pre-treatment of the uncoated granule surface. Alternatively, an initial primer coating can be applied. Since the primer coating itself is unsatisfactory for slow release purposes, an outer encapsulating coating must then be applied.
An additional problem encountered with conventional techniques of coating granules is achieving a uniform coating thickness. When the coating material is sprayed onto the granule, much of the material lands on the bare granule surface while only some of it lands in the pore containing the pesticide or other liquid whose delayed release is desired. The thickness of the coating varies according to the spraying technique used and the angle at which the spray strikes the granule. Some of the coating material will be wasted on the dry granule surface where it has no slow release effect. Some areas of exposure of the liquid contained in the granule pores will be coated very lightly, allowing too much liquid to escape. Other important areas may not be coated at all. This lack of uniformity arises in part from the fact that the coating composition is not premixed with the pesticide in a single body of liquid prior to its application to the granule. The efficiency of the coating material thus applied and the reproducibility of the process are less than desired.
It is therefore an object of this invention to overcome the problems stated above and provide a process for the preparation of porous granules impregnated with a liquid material and sealed with an encapsulating coating which uniformly seals the liquid material inside the pores and provides a controlled rate of release.
Another object of this invention is to provide a novel impregnated granule with pores uniformly sealed by a single uniform coating capable of providing a controlled rate of release of the liquid contained in the pores.